1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system and method for exciting magnetic resonance in a specific portion of an object to be examined, which is placed in a static magnetic field, by applying a gradient magnetic field and high-frequency pulses (RF pulse) of a 90.degree. pulse-180.degree. pulse sequence to the object, acquiring spin echo signals induced by the magnetic resonance, and imaging the specific portion by using the spin echo signal data in accordance with a predetermined image reconstruction method.
2. Description of the Related Art
In a general medical MRI system, a gradient magnetic field and a high-frequency pulse are applied to an object to be examined, which is placed in a static magnetic field, in accordance with a predetermined sequence for magnetic resonance (MR) excitation/magnetic resonance (MR) data acquisition. As a result, an MR phenomenon occurs in a specific portion of the object. An MR signal induced by the MR phenomenon is detected. When acquired MR data is subjected to imaging data processing including image reconstruction, the anatomical data or quality data of the specific portion of the object is imaged.
A conventional MRI system of this type generally comprises a static field generator, X-, Y-, and Z-axis gradient field generators, and high-frequency transmitter and receiver. When the X-, Y-, and Z-axis gradient field generators and the high-frequency transmitter are driven in accordance with a predetermined sequence, X-, Y-, and Z-axis gradient fields Gx, Gy, and Gz, and a high-frequency pulse are generated. As a result, MR is excited and an MR signal is generated. The MR signal is received by the receiver. The reception data is then subjected to predetermined image processing including image reconstruction. The tomographic image of a certain slice portion of an object to be examined is generated in this manner, and is displayed on a monitor.
In an excitation sequence of MR, the X-, Y-, and Z-axis gradient fields Gx, Gy, and Gz are respectively used as, e.g., a read gradient field Gr, an encoding gradient field Ge, and a slicing gradient field Gs.
As a conventional MR imaging method often used in such a system, an imaging method based on a spin echo sequence using high-frequency pulses of 90.degree. pulse-180.degree. pulse sequence is known.
Such a conventional spin echo sequence will be described with reference to FIG. 1.
In the spin echo sequence, in order to minimize influences due to inhomogeneity of a static field, the relationship between time t=0 when a 90.degree. pulse is applied, time t=T180 when a 180.degree. pulse is applied, and time t=TE when the peak of a spin echo signal appears must satisfy the following condition: EQU T180=TE/2
In this case, the earliest time when echo signals can be acquired comes after application of a 180.degree. pulse having a pulse width Tw. In addition, this start time comes at a point A in FIG. 1, i.e., time at which the leading edge of the reading gradient field Gr is stabilized at the end of the trailing edge of the slicing gradient field Gs. Moreover, when data acquisition is performed in a symmetrical manner with respect the echo peak at time t=TE, an echo signal acquisition time Taq is limited to the following condition: EQU TE-Tw-2.alpha..gtoreq.Taq
where .alpha. is the fall time of the slicing gradient field Gs and of the rise time of the read gradient field Gr. If the resolution is kept unchanged, the upper limit of the time Taq is determined by TE, Tw, and .alpha., and moreover, the intensity of a gradient field cannot be satisfactorily decreased. Since ##EQU1## the signal-to-noise ratio (S/N) cannot be satisfactorily increased.
As described above, in the conventional system, a 180.degree. pulse is applied while a time 1/2 the echo time TE (a time interval between the peak of a 90.degree. pulse and that of an echo signal) is set to be T180, i.e., EQU TE/2=T180
For this reason, the upper limit of the echo signal acquisition time Taq is limited by TE-Tw-2.alpha..gtoreq.Taq, and an increase in S/N is limited when the resolution and the TE time are kept unchanged.
Recently, however, with an improvement in technique for obtaining a homogeneous static field in an MRI system, inhomogeneity of a static field can be reduced to such a degree that no problem is posed in practical applications.